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Rock Wren

Rock wren (hurupounamu) are small insect-eating birds which live at altitude in the South Island. The New Zealand wrens are an extremely significant group of birds, being the only members worldwide of the infraorder Acanthisittides. Sadly, of the half dozen New Zealand wrens known to have existed, all but two are extinct. The bush wren and the Stephens Island wren have died out since European colonisation of New Zealand. The Stephens Island wren was wiped out by the lighthouse keeper’s cat in the late 1800s and the bush wren was done for by an invasion of rats onto its only remaining site in 1964. Of the remaining two, the rifleman is listed as Declining and the rock wren is officially Nationally Vulnerable. Rifleman can be seen in a lot of places, but many people will never see a rock wren. We have made rock wren a major priority and this work has been our biggest spend on the public conservation estate.

Rock wren are attracted to feathers for nest material and chicken feathers can be placed to help track rock wren to nests. Photo: Sue Heath

We don’t know for sure how many rock wren are left, but anecdotal evidence has long suggested that their range is shrinking. Several characteristics of rock wren make them vulnerable: they nest in holes making them easy prey for stoats; they tend to nest only once in a season; they are short-lived; and they fly weakly and don’t tend to move between areas. These factors combine to give a species which could become extinct quite quickly, site by site, and largely without us noticing.

Getting Info on Numbers and Range

There is a big information vacuum about rock wren populations and their current range. Survey work was required and the first attempt was a walk-through survey of an area in Kahurangi National Park where there were many historical records of rock wren. One of New Zealand’s most experienced bird people found only a single rock wren in 8 days.

Another study managed by Sue Heath and Peter Gaze correlated rock wren sightings with the age of the record: pre-1984 records showed rock wren spreading right over the main divide and foothills of the South Island, but recent records were more patchy and seem to be shrinking closer to the main mountain range.

10 km X 10 km squares where rock wren have been reported. Lighter squares are where there have been no sightings since 1984. Map: Sue Heath and Peter Gaze

The map looks pretty compelling but we recognise several uncertainties with the approach taken, especially the possibility of less effort put into sightings more recently now that conservation staff no longer conduct species surveys as a regular part of their work.

Research in the Murchisons

The Murchison Mountains are a relative stronghold for rock wren and thanks to the infrastructure surrounding the effort with takahe there, are quite accessible. Rock wren heroine Megan Willans wanted to build on earlier work by Sue Heath and discover basic facts on what was happening to rock wren on the mainland, and get clues on how to protect them.

First up, Megan and her team developed (2005-7) population survey techniques in the McKenzie burn area, which had been studied by Sue Heath in 1984-5. They found that the population had declined by 44% in just over 20 years- and this is a stronghold.

The team also wanted to get evidence of what predators were causing most problems for rock wren, and 2006-7 was a mast year for tussock, which was expected to lead to more predators being present. They monitored 16 nests, and two nests were predated. Mice and stoats were detected commonly in the area over summer. Indications were that the rock wren had moved to suboptimal habitat and were nesting on inaccessible bluffs to avoid predators. This is possibly why the area remains a relative stronghold despite not receiving mustelid trapping.

Conservation is not an exact science and these results are far from a smoking gun. However we were not of a mind to wait until evidence of precipitate decline was incontrovertible and rock wren were pitched into the basket case category so well represented in New Zealand’s land fauna.

Anchor Island Translocation

There is little alpine habitat suitable for rock wren on our predator-free islands, but the time to try for some insurance populations is now, before the situation becomes critical.

Catching with mist net in the Murchisons. Birds were on Anchor within 3 hours of catching. Photo: DOC

In early 2005 Anchor Island was the only pest-free island with any alpine habitat. The translocation of 28 rock wren from a healthy mainland population was highly experimental, with best guesses having to be made about when to transfer after breeding. It was decided to transfer families of adult pairs and fledglings as soon as they were strong enough after breeding season and well before winter.

A survey of part of the island immediately after transfer turned up quite a number of the birds. We were quite optimistic knowing that they found enough food and shelter to at least survive short term. If the habitat were entirely unsuitable such small birds would have all died in the first couple of days after release.

A survey after a month showed the birds in the predicted places, but unfortunately they did not establish. Map: Megan Willans, DOC

Unfortunately the transfer is now known to have been unsuccessful, with later surveys indicating that probably only a couple of birds survived for a year. The catch site on the mainland filled up again quickly enough so at least we knew we could attempt another translocation.

Secretary Island Translocation

Secretary Island, at 8140 hectares and now with stoats and deer at very low levels since trapping began in 2005, has become an important island refuge for threatened species. Importantly, there are some good alpine areas for rock wren.

Stoic rock wren being fitted with colour bands. Photo: DOC

In Autumn 2008 Megan Willans and her colleagues at DOC Te Anau translocated 9 of a projected 40 rock wren to Secretary. The target of 40 was to give good genetic diversity and birds were to be caught from several areas in the Murchisons. The work was exacting, requiring good weather for bird mist-netting and helicopter operation. The birds were kept in boxes for a maximum of 2.5 hours and hard-released on Secretary near the top of the island. The timing of transfer was changed compared with the Anchor translocation, in order to allow the adults to recover from the breeding season, but before the weather deteriorated.

It might not be home, but it’s free of stoats. Photo: Megan Willans, DOC

Megan was delighted to report that 4 of the 9 were re-sighted during a quick monitoring trip in late 2008. She also saw one of the females carrying nest material. The remaining birds were moved over the following two years- total translocated: 40. In 2010 we received a very happy message, a survey team reporting 12 unbanded birds (bred on Secretary Island) and a reasonable number of the founder population. This is the first successful translocation of rock wren.

Bird translocation is not something to be taken lightly. Proposals require extensive review and sign-off, and catching, care, transport, release and monitoring need meticulous planning. Make no mistake, sentencing birds to transportation for life for the good of their kind is a difficult decision. Release day is invariably an emotional occasion.

Megan Willans releases rock wren near the top of Secretary. Photo: DOC

Funded by Fauna Recovery New Zealand ($120,000). Great support from Fiordland Helicopters with some of the cheapest helicopter time on the planet billed to the project (Kim, we did notice!). Main photo courtesy and Copyright Sue Heath.